How to caramelise onions: The art and science of getting it right

Patience is key to unlocking that rich, sweet flavour which makes caramelised onions so special. Photo / Getty Images

Caramelising onions transforms a humble ingredient into something rich, sweet and deeply flavourful. The process may seem straightforward, but achieving perfectly caramelised onions requires patience and a few key techniques. Here’s everything you need to know.

The science behind caramelisation

Caramelisation occurs when the natural sugars break down and brown over low heat, resulting in that signature golden hue and sweet taste. This process takes time - typically around 40 minutes to an hour - but the depth of flavour is worth the wait. The key is to cook the onions slowly, allowing the sugars to develop without burning.

How to make caramelised onions

1. Start with a large pan, preferably stainless steel or cast iron, which distributes heat evenly.
2. Slice your onions thinly for quicker cooking, but not so thin they burn easily.
3. Add a generous amount of butter or oil to the pan and heat over medium-low. Once the butter is melted and slightly foamy, add the onions, stirring to coat them evenly.
4. Let the onions cook, stirring occasionally. After about 20 minutes, you’ll notice them starting to brown. This is when you need to pay close attention. If the onions stick or start to burn, lower the heat and add a splash of water to deglaze the pan.
5. Continue cooking until the onions reach a deep brown colour and have a jam-like consistency.

Avoid the temptation to rush the process by turning up the heat - high temperatures can burn the onions before they’ve had a chance to properly caramelise. Remember, patience is key to unlocking that rich, sweet flavour which makes caramelised onions so special.

Once caramelised, onions can be stored in the fridge for up to a week or frozen for longer periods.

Additional flavours

Balsamic vinegar: Add a splash during the last few minutes of caramelisation. Stir it in and allow it to reduce slightly, which will intensify the sweet and tangy flavours.

Thyme or rosemary: Add fresh thyme or rosemary sprigs about halfway through the caramelisation process. This gives the herbs enough time to infuse their flavour into the onions without overpowering them.

Garlic: Add finely chopped garlic during the last five to 10 minutes of caramelisation. Garlic burns easily, so adding it late in the process allows it to soften and sweeten without becoming bitter.

Wine or sherry: Deglaze the pan with wine or sherry towards the end of the cooking process, when the onions are almost fully caramelised. Pour a small amount into the pan and stir, scraping up any browned bits from the bottom of the pan.

Brown sugar or honey: Add brown sugar or honey in the last 10 minutes of caramelisation. This gives the sweetener time to melt and blend with the onions, creating a deeper caramel flavour.

Soy sauce: Add soy sauce during the final few minutes of cooking. This allows the soy sauce to mix with the onions and deepen their umami flavour without overpowering the natural sweetness.

Cider or apple juice: Add a splash towards the end of the caramelisation process. Allow it to reduce slightly, which will enhance the fruity notes and blend with the onions’ natural sweetness.

Chili flakes: Add chili flakes during the last few minutes of cooking, depending on how much heat you want. Stir them in well so the spice is evenly distributed without overpowering the onions.

Each of these additions should be incorporated thoughtfully to enhance the onions without overshadowing their natural flavour.

Uses for caramelised onions

Burger topping, toastie filling, savoury tarts and quiche ingredient, accompaniment for steak, pork, chicken and sausages, chopped and mixed into sour cream with chives for a tasty dip, added to hummus with a drizzle of olive oil, used with mayo for a tasty sandwich addition - these onions are only limited by your imagination.

Herald contributor Nikki Birrell has worked in food and travel publishing for nearly 20 years. From managing your kitchen to cutting costs, she’s shared some helpful advice recently, including how to use a fresh artichoke, getting the most out of vinegar, and whether spreads, condiments and chocolate go in the fridge or pantry.